When starting up a steam turbine, the temperatures in the exhaust steam region of the high-pressure turbine can exceed permitted levels if the steam turbine is operated under little or no load, and little or no electricity is fed into the consumer grid. In order to lower the temperature, two measures are potentially conceivable: 1.) lowering the back-pressure of the high-pressure turbine; 2.) increasing the mass flow through the high-pressure turbine
However, in start-up operation, under little or no load, it is not possible to increase the mass flow since this would increase the turbine output. For that reason, prior art steam turbines have, for start-up operation, what is termed a start-up line which connects a region downstream of the high-pressure turbine (also termed exhaust steam space) to the condenser of the steam turbine, and thus makes it possible to lower the back-pressure of the high-pressure turbine.
In prior art steam turbines, switching from start-up operation or no-load operation to output operation involves closing the start-up line. It is necessary to close the start-up line because the mass flow of the steam which is fed to the condenser via the start-up line is not available for cooling the reheat.
Closing the start-up line raises the pressure at the outlet of the high-pressure turbine and thus the outlet temperature of the high-pressure turbine. An impermissible temperature rise after closing of the start-up line can be prevented by simultaneously increasing the mass flow through the high-pressure turbine.
In that context, closing the start-up line too rapidly leads to pressure fluctuations in the water/steam circuit, which can even lead to emergency shutdown of the turbine.
Increasing the mass flow through the high-pressure turbine too slowly during closing of the start-up line leads to impermissibly high temperatures in the exhaust steam region downstream of the high-pressure turbine.
These two conditions require the closing of the start-up line and the opening of the live steam valves to be optimally reconciled in order on one hand to be able to rapidly increase the mass flow of the steam via the live steam valves, and thus to keep the temperature low, and on the other hand to limit the mass flow via the start-up line in order that the reheat is sufficiently supplied and a so-called high-pressure redirection station can adjust the live steam pressure.
Hitherto, this object has been achieved by quick closing of the start-up line and anticipated control to the high-pressure redirection station. However, this approach leads to a highly transient temperature in the exhaust steam region of the high-pressure turbine, and to a highly transient mass flow in the turbine, in the live steam line and in the line to the reheat.
WO 2013/031121 A1 discloses a steam turbine device and a method for operating same, wherein start-up of the steam turbine is controlled by means of an overflow line system.